1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon processing with a fluidizable catalyst. More particularly, in one embodiment the present application relates to a hydrocarbon feed distributor and, in another embodiment, to a particular method of injecting a hydrocarbon feed into a catalyst conversion zone, both of which find particular utility in the fluidized catalytic cracking process.
2. Description of the Prior Art
The catalytic cracking of hydrocarbon feedstreams is well known. The fluid catalytic cracking (FCC) process wherein the present invention finds particular applicability comprises mixing in a riser reaction zone a hydrocarbon feed having a boiling range of from about 500.degree. F. to about 1200.degree. F. with a fluidizable catalyst and converting therein, at conversion conditions, the hydrocarbon feed into lighter, more valuable, products. Typically the temperature of the hydrocarbon feed is from about 350.degree. F. to about 700.degree. F. and the temperature of the regenerated catalyst is from about 1150.degree. F. to about 1350.degree. F. The two are mixed together to completely vaporize the hydrocarbon feed and to achieve a temperature within the conversion zone of from about 875.degree. F. to about 1100.degree. F. Conversion conditions also typically include low pressures of from about atmospheric pressure to about 100 psig and hydrocarbon residence times of from about 0.5 second to about 5 minutes. Catalyst is normally circulated through the riser reaction zone at a rate of from about 4 to about 20 pounds of catalyst per pound of hydrocarbon feed. The catalyzed reactions may be conducted entirely in a riser reaction zone, as in an all-riser FCC unit, or partially in a riser reaction zone with the mixture of catalyst, reaction products and unconverted feed, if any, then being discharged into a dense bed of fluidized catalyst for further conversion of the feed or of the heavier reaction products into lighter reaction products. The apparatus and method of this invention find utility in either case.
A variety of techniques have heretofore been employed to introduce a hydrocarbon feed into the riser reaction zone. U.S. Pat. No. 3,152,065 for example describes a method of injecting a hydrocarbon feedstock into a catalytic reaction zone which comprises passing the liquid hydrocarbon as an outer stream in a generally linear direction, imparting a centrifugal energy component to the outer stream, passing the outer stream having a centrifugal component through an annulus, and discharging the moving stream through a restricted passageway in contact with an inner stream of a vaporous material such as steam which operates to disperse the hydrocarbon stream into small droplets of liquid. Other vaporous or gaseous materials such as inert gases, nitrogen, natural gas, recycle catalytic cracking unit process gases, etc. can be used as the inner stream. Also disclosed in that same prior art reference is a nozzle for injecting a liquid hydrocarbon feed into contact with a catalyst which nozzle contains components for imparting a centrifugal energy component to material flowing through an outer shell of the nozzle. Both the method and the nozzle are for providing a high degree of atomization of the feedstock and good contacting of the hydrocarbon feedstock and the catalyst. This high degree of atomization is achieved by the use of means of imparting a centrifugal energy component to a liquid hydrocarbon stream and by using a "vaporous material" which operates to disperse the hydrocarbon stream into small droplets. U.S. Pat. No. 3,654,140 describes an improved catalytic cracking process which comprises feeding a substantially liquid hydrocarbon oil feedstock to at least one feed injection zone of a fluidized catalytic cracking reaction zone, concurrently feeding steam to said injection zone in a volumetric ratio of steam to liquid hydrocarbon ranging from about 3 to about 75, thereby imparting to the resulting mixture an exit velocity relative to the fluidized catalyst of at least about 100 feet per second, whereby the oil feedstock is essentially completely atomized forming droplets less than about 350 microns in diameter. The process of this reference relies on the use of steam and very high exit velocities of at least 100 ft/sec to achieve a high degree of feedstock atomization characterized by droplet sizes of less than about 350 microns in diameter.
These prior art processes and apparatus and others have been primarily concerned with the initial contacting of the hydrocarbon feedstock and catalyst to achieve, at least initially, a uniform mixture of catalyst and hydrocarbon feed in the riser reaction zone to avoid excessive coking of the feedstock and attendant product loss. While the initial formation of a uniform catalyst and hydrocarbon mixture is certainly important, it is equally important that the mixture uniformity be maintained as well as possible across a cross-section area of the riser reaction zone at any elevation along the riser reaction zone. More specifically, it has been found that in spite of the use of methods and apparatus to achieve initial uniform contacting of a hydrocarbon feed and a cracking catalyst, wide variations of catalyst density and temperature can exist across cross-sections of typical riser reaction zones, particularly across cross-sections at lower elevations of riser reaction zones. With the use of radiation equipment and probes containing thermocouples, catalyst densities and temperatures in riser reaction zones at different elevations have been measured and catalyst density and temperature contours have been obtained. At lower-elevation cross sections of a riser reaction zone catalyst densities of about 60 lb/cu ft. have been found near the walls while catalyst densities of less than about 3 lb/cu ft. were found on the same cross section but near the riser centerline. Temperature profiles have shown the same wide variation; at lower-elevation cross sections of a riser reaction zone temperatures of 1200.degree. F. and higher have been measured while temperatures near the centerline of the riser were about 650.degree. F. Such high wall temperatures cause elongation of the riser reaction zone and in many instances exceed the design wall temperature and result in permanent deformation of the riser reaction zone. Additionally high wall temperatures cause overcracking of hydrocarbon feed in these regions of higher temperatures and result in increased yields of dry gas (C.sub.2 -). The apparatus and method of my invention reduce these high wall temperatures and the problems they cause. My apparatus and method produces temperature profiles across a cross-section of a riser reaction zone which are more nearly flat thus reducing overcracking and the risk of damage to the riser reaction zone.